Casehardening composition



Patented Dec. 27, 1949 2,492,805 CRSEHARDENING? COMPO SI'FION PaulLeiningiRGrand Island, N2 Y., assignor tojE. I. dilfPbilt" d'e Nemoms & Cdmpahy, Wilming'ton, Deli; a corporation of Delaware- NmDraiwingi Application December This invention relates to case-carburitation er 's'teel articles by'treat'mentiin' molten cyanide ease hardening baths and'ha'sasan oniecttne iiierease in carburizingactivityof such baths.

This is a continuation in part of my depending' application Serial Nb. 656,766 filed March to rapidly obtain a deep, carburi zed case, with I little or no nitri'ding. The most successful results have been obtained by including alkaline earthmetal salts'in the melt; However, lfneco'r'icentration of" alkaline: earth metalisaits. required to successfully accomplish this object is large; consequentlythe cyanide" replenishing. agent used to maintain thebath'atnolma'l'activity nineteen"- taint in addition is the cyanide" large perce tages of" alkaline eartlimetailsalt's, S'O' that a h 66hcentrationo'f these Will be maintainedin the bath.

Because ofthis, it isneces'sary'to periodically-tail out large parts of the" bath in order to be stile to replenish it with fresh salts. This is unecdfi'ofii' addition; alkaline earth activated baths produce water insoluble" residues on the treated Work, interfering With the. subsequent cleaning operations;

This'inventio'n has as an objecttd provide an improved proc'ess'for case hardening ferrous-arti'cl'es: A further object is to provide a process for accelerating the carburizingaction of molten cyanide case hardening" baths; A further object is to provide an accelerated cyanide case hardening' bath free from constituents that are orwill form water insoluble inatei-ialsupon Wdrktieated therein, i. e., an accelerated cyanide. ease hardening'bath' with easy washing chara is'tis' A further object is to provide an acne eiated ype cyanide case hardening bath Wliili' Will i'niiiii'iiifi'e o1? eliminate: the: exeessive'rb'aileout-ofz loath! which now: necessary when replenishing. the. caste mary alkaline: earth activated: type: accelerated bathe Anotherv obj ect isito": providea-nnvelieompu sitions usefuli for: replenishing: the bath: on this invention .withoutibailingr. out. and? withoutt other additionsi of batli= ingredientsi. stiilf further oiijet is 116* ere-vise air accelerated cyanide case nanny carboniii compesitlbns Qther objects will be apparent from ther tol-lowing disclosure.

The above objectsmay be attained-in accordance my, invention by ad'd'ifiit'o a carbonate co'nt'ainihg aye re oxygen (Loin of boron: or sil on anddisfiersirigfiiielyf'divided carton" ilriroiig iit 't-YieQBatfiZ For example; the addition of 2% by? weight ofb'qroi'l oxide; (Blot; and $i6ess raphifi tiia standa'i' d cyanide c e hardening? bath coliit aiiiingi fioifmal concentratioiji" ereyaniee andlcai bfi'na'te (i1 (5;, a bath emiriio'sed of alkfafifinetal e taiiiii'es, carbonates and halides; but not containing, any eatalys'ts to at"- cl'erat case hardening, seen as forlexalfiple, the alkaline'aiitli ifietal'salts'; andiiiavmg a cyanide eonte'ntoffioto 46%, and a earlionate eententor 10 to 80%), produced a marked increase in the carburi'zing action oftli bath and'fiads'iibstaiisane. no efi'edt on its nitridingi activity. Adding 5% of boron oxide produced an even more drastic increase-in the baths carburizing activity. Addition ofaboronoxideand carbonto asimilai: cyf amide-carbonate" bath" containing less' than 20% of. cyanide, als'oresulted ina drastic increase in th'e'b'atlis carburizing' activity; Prolonged operation of the batlrat 1550 resulted in a-sligiit falling elf ofits activity; thusneeessitating small periodic additions of boron oxide to the bath to maintain-dis tulLacti-vitsp v I likefollovv'ingm examples-further serve ta-illus trate my; invention 2 semester Acylanid'e bathicofitainiiig an iifox'ii ltiately.-

Sodi-iirii cyanide, 375%- sddiuni carbonate and 37.5%. sodium efiioride. was lia't'd'; to 843" c:

(1550 133. Excess newde'rfed rapiiite next added uritill iayerateui 1 thiliffemairied or'i 156p ef'ti'ie molt n bath. n sire'xrom st bar 07' long afid'i"""iii di'afiiete'i)" was immersed in the bath and treated for 1" hour, then removed from the bath and air cooled-': An addition of 2% of the. bathweight. of boric acid. anhydride was next added to the molten bath. A second SAE X1020 steel testbar was-treated-for Lhoui By immersin iii the bath, following" which it wasrem'oved and cooled; The twosteeltesfi bars were washed free from adfi'e'fifig" Salt six successii e 1ayers-0.-004 deep-were-cut'froni the" periphery-- ofeach: The metalsamples thus a v 3 obtained were analyzed for both carbon and nitrogen. The results were as follows:

g. ttt ibiti Out N o. 6 rap 1 e and 2% Percent Percent N Percent 0 Percent N Units 56 32 109 65 1 Core.

Carbon and nitrogen units given in the above and succeeding tables herein are a measure of the amounts of carbon and nitrogen taken up by the steel. The carbon units are calculated by subtracting the percentage of carbon in the core from each of the percentages of carbon found in the successive 0.004 radial cuts, adding the resulting figures and multiplying the sum by 100. The nitrogen units are calculated in the same way, using the results of nitrogen analysis.

Example 2 A cyanide bath containing about 33 sodium cyanide, 33 A; sodium carbonate, and 33 V; sodium chloride was heated to 843 C. (1550 F.). Excess powdered graphite was added until a layer about /4" thick remained'on top of molten bath. About 5% of the bath weight of boric acid anhydride was added to the molten bath. A bar of SAE X1020 steel (similar to those described in Example 1) was treated by immersing for 1 hour in the molten bath. After the bar was removed, air cooled, and washed, successive 0.004" deep cuts were removed from the surface for carbon and nitrogen analysis. The results obtained were as follows:

Per Cent Carbon Per Cent Nitrogen sodium nally, 5% of the bath weight of boric acid anhydride was added to the bath. A bar of SAE sodium X1020 steel was treated for 1 hour in the bath in a fashion similar to that described in Examples 1 and 2. Carbon and nitrogen analyses of the gradient cuts removed from the surface of the test bar showed the following results:

In practicing my invention, I utilize carbonatecontaining baths of molten alkali metal cyanide. e. g., cyanides of sodium, potassium, lithium or other alkali metals, with or without other alkali metal salts. I prefer to utilize mixtures of one or more alkali metal cyanides with an alkali metal halide (e. g., chloride, bromide or iodide) and an alkali metal carbonate, containing about 5 to 40% by weight of cyanide. I disperse in the melt not less than 1% by weight, and generally not more than 15% by weight, of finely divided carbon. This is best accomplished by adding an excess of the carbon to the bath, so that in addition to that dispersed, there is a layer of carbon floating on the baths. Any desired form of free carbon can be used, graphite being generally preferred. 1 also add to the bath 1 to 20% by weight of an oxygen compound of boron or silicon, preferably boron oxide (B203) In place of boron oxide, I may use other oxygen compounds of boron or silicon which are more acidic than alkali metal carbonates, for example, alkali metal borates, silica or alkali metal silicates.

The bath must contain at least one mole of carbonate for each mole of boron oxide or its equivalent added to the bath, and preierably a 5% excess over that amount. Larger amounts of carbonate may be used, as desired. For example, a bath consisting of sodium carbonate with 10-30% by weight of sodium cyanide and 1 to 20% by weight of boron oxide is suitable.

Steel articles are carburized according to my invention by immersing them in the bath, which is maintained at a temperature within the range of 1450 to 1850 F.

Since there is a slight decrease in carburizing activity when the bath is operated over a prolonged period of time, it will be necessary to replenish it periodically with fresh boron oxide or its equivalent, while maintaining a sufiicient excess of carbon in the melt. Periodic replenishments of cyanide-rich material will also be necessary to maintain the cyanide content of the bath at 540%. Fresh carbonate may have to be added if there is an insufficient amount formed from the decomposition of the cyanide, but generally carbonate additions are not required, except as may be required to compensate for dragout. The bath can be operated for prolonged periods of time with much less bail-out than with the alkaline earth metal salt activated cyanide baths.

For commercial operations, where more or less drag-out occurs, it is desirable to have a replenishing mixture which can be added to the bath periodically and which will maintain the bath ingredients in suitable operating proportions as described above. A replenishing composition which is well adapted for most commercial operations of the present invention has the following approximate composition, in which the amounts are expressed in percent by weight:

Per cent Sodium cyanide Boron oxide (B203) 10 Sodium chloride 15 Sodium carbonate 4 Graphite 1 All of the above ingredients are in finely divided form, e. g., flaked, granulated or powdered, and are thoroughly mixed together. The mixture may be stored or shipped over long periods of time and under various conditions without substantial change, until ready for use. In most cases, this mixture can be added to the bath as required to compensate for drag-out, without having to either bail out any of the bath or to add other substances; and such addition to the mixture will maintain the bath in operative condition.

While the abovedescribed mixture is suitable for replenishing the bath in the majority of cases, some variation may be required in some instances, for example, where drag-out is greater or less because of size and quantity of articles treated in the bath. In general, such variation in the properties of the ingredients of the replenishing mixture will fall within the ranges shown in the following table, wherein quantities are expressed in parts by weight:

Per cent Sodium cyanide 40 to 70 Boron oxide 5 to Sodium chloride 15 to 50 Sodium carbonate 3 to 13 Graphite 0.5 to 3 In place of the boron oxide, the above replenishing mixtures may contain a stoichiometrically equivalent amount of other oxygen compounds of boron, or of silicon, which are more acidic than alkali metal carbonates. In place of the graphite I may use other forms of carbon. Preferably, all ingredients are substantially anhydrous.

An important feature of the above described replenishing mixtures derives from the relative proportions of sodium carbonate and boron oxide. These proportions are such that when the mixture is dissolved in water, the resulting solution is strongly alkaline. Preferably, in any mixture, the proportions are so adjusted that the water solution has a pH of about 10 or higher. This feature insures that poisonous hydrocyanic acid will not be evolved, should the mixture accidentally come into contact with water. Thus, the mixtures may be handled safely under various mill and shipping conditions.

One of the advantages of the process is that it permits one to obtain accelerated case hardening in a molten cyanide bath by increasing the carburizingaction of the melt instead of its nitriding activity. This is of particular value for those applications in which high nitrogen cases are detrimental. A second advantage of the process is that it permits one to obtain accelerated case carburizing in a cyanide-containing bath without introducing compounds which make subsequent washing of the treated parts difficult. Another advantage of the process is that it permits accelerated case carburizing without necessitating the bail-out of large quantities of the operating bath when making replenishments, as is necessary with some alkaline earth activated type accelerated cyanide case hardeningbaths.

I claim:

1. A case hardening bath replenishing composition consisting of the mixture, in finely divided form, of the following ingredients:

Per cent by weight Sodium cyanide 40 to Sodium chloride 15 to 50 Sodium carbonate 3 to 13 Carbon 0.5 to 3 and an oxygen-containing compound of boron more acidic than alkali metal carbonates in an amount stcichiometrically equivalent to 5 to 15% by Weight of boron oxide.

2. A case hardening bath replenishing composition consisting of the mixture, in finely divided form, of the following ingredients:

Per cent by weight Sodium cyanide 40 to 70 Sodium chloride 15 to 50 Sodium carbonate 3 to 13 Graphite 0.5 to 3 Boron oxide 5 to 15 3. A case hardening bath replenishing composition consisting of the mixture, in finely divided form, of the following ingredients:

Per cent by weight Sodium cyanide, approximately 70 Sodium chloride, approximately 15 Sodium carbonate, approximately 4 Graphite, approximately 1 Boron oxide, approximately 10 PAUL M. LEININGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,736,457 Merten Nov. 19, 1929 1,804,454 Beck May 12, 1931 1,980,152 Beck Nov. 6. 1934 2,095,188 Hanusch Oct. 5, 1937 2,210,622 Keil Aug. 6, 1940 2,339,223 Holt Jan. 11, 1944 OTHER REFERENCES Molten Salt Baths, page 7-9, published by E. I. du Pont de Nemours & Co., Inc., 1942. 

